Over most of 2018, oil rebounded considerably from the previous year, hitting the $70s/barrel by summer. But by December, oil prices had once again begun to fall, as rising U.S. stockpiles and falling economic sentiment helped depress demand.

It may seem counterintuitive, but refiners and other services companies actually benefit from falling oil prices. Refiners have to purchase crude to supply their factories, so lower oil prices mean higher margins for them.

Meanwhile, pipeline companies benefit from the volume of oil that passes through their infrastructure; lower oil prices more crude oil flows.

Furthermore, equipment sellers can benefit from a slide in oil prices, as wildcatters often take that opportunity to pause their exploratory activities to upgrade equipment and better position themselves for a pricing rebound.

Overall, XES' portfolio has a weighted average market cap of $4.9 billion, compared with OIH's $18 billion average market cap. XES also has more names in its portfolio than OIH (36 holdings to OIH's 23), resulting in a more diverse take in a field of relatively top-heavy products.

With an expense ratio of 0.35%, XES costs the same as OIH, the segment's heavyweight. It is also fairly liquid, with strong average volume ($19 million traded daily) and relatively tight spreads of 0.10%.